The invention relates to an apparatus for hot-pressing a planar product in a process using microwave heating.
Heating of planar wood products containing glue layers up to the curing temperature of the glue using microwave heating is known in the art. Apparatuses based on this heating technique have the heating unit formed into a box-like resonant cavity via which the product to be heated is passed. The product is passed via a narrow gap that extends lengthwise through the entire cavity and in which gap the product travels conventionally supported by conveyor belts. The material of the belts is selected to be transparent to microwave radiation.
After the heating step, the product is generally passed to a press apparatus, wherein the throughout heated product is pressed to its target thickness and the glue is cured by the latent heat of the product.
Conventional processes have rarely used pressing during the heating step due to the limitations set by the basic construction of the microwave heating system. Microwave energy must be contained in a resonant cavity through which the product being heated is passed. To establish resonant conditions in the cavity, its walls must be spaced at a given distance from each other. This constraint excludes the possibility of moving the parts of the resonant chamber in regard to each other in order to accomplish the pressing step. A certain degree of compressive pressure has been imposed in the prior art on the product during the heating step by way of arranging press rolls on the opposite side of the conveyor belts traveling through the resonant cavity such that the rolls have been able to press the conveyor belts against the product being heated. This embodiment, however, is limited by the belt material durability as to the maximum applicable pressure and, moreover, to the uniform distribution of the pressure imposed over the product surface. The belt material must be selected transparent to microwaves, whereby the use of metallic belts for instance is excluded.
Now, an improvement according to the invention is offered to ovecome such problems by virtue of an apparatus comprising in a conventional fashion a constant-volume, box-like microwave resonant cavity suitable for division into two opposed subcavities by an elongated gap suitable for passing therethrough the product to be heated and conveyor belts of microwave-transparent material for guiding the product to be heated through the gap, whereby the subcavities are delineated from the conveyor passageway gap by parallel walls of microwave-transparent material, each one of them sealed so as to form a wall of its own subcavity, of which subcavities at least one is pressurized and that the sealed wall of the at least one pressurized subcavity is designed to yield under the pressure applied into the subcavity in a direction perpendicular to the product to be pressed.
Such pressurization of at least one of the opposed subcavities of the resonant cavity makes it possible to apply to the conveyor belts a moderate, uniformly distributed pressing force that allows the product to be pressed in a desired fashion already during the heating step of the product.
Next, the invention will be examined in greater detail by making reference to the appended drawing, wherein an apparatus according to the invention is shown in a sectional side elevation view.